Hot melt pressure sensitive adhesives (HMPSAs) including a styrenic block copolymer (SBC) have been used for various applications, such as for tapes, labels, product assembly, and hook and loop applications. Due to the industry requirement for their shear adhesion performance, HMPSAs for these applications have traditionally contained styrene-isoprene-styrene (SIS) type block copolymers with high copolymer content. High copolymer content provides high shear adhesion performance, but also leads to high viscosity at application temperatures, and more recently, higher adhesive costs for SIS based HMPSAs.
It has been a challenge in the art to formulate a styrene-butadiene-styrene (SBS) block copolymer based HMPSA that has comparable performances relative to a similar SIS based HMPSA.
SBS block copolymers tend to have higher viscosities relative to the SIS block copolymers. Therefore, a SBS based HMPSA that has a high copolymer content to achieve the desirable high shear performance often exhibits even higher adhesive viscosity relative to that of a conventional SIS based HMPSA.
A high viscosity HMPSA is difficult to apply to substrates, especially at the high production rates preferred by the industry. Attempts to increase application temperatures can lead to the copolymer degradation and the decreased adhesive pot life, which reduces the ultimate adhesive performance, let alone the increased energy consumption, hazardous plant environment due to smoking and deformation of heat sensitive substrates. Using block copolymers with higher levels of styrene generally reduces the adhesive viscosities, but the HMPSAs are less flexible, and lower in tack properties. Specific adhesion attributes can also be affected. Using block copolymers with higher diblock content may also help to lower the adhesive viscosities but at the expense of greatly decreased cohesive strength, and thus, the reduction in shear adhesion performance. Liquid plasticizers are very efficient in lowering the viscosity of a HMPSA, but adversely affect the strength and shear performance.
Therefore, there is a need for a HMPSA including a styrene-butadiene-styrene (SBS) block copolymer that has not only high shear adhesion performance, but also a low viscosity suitable for applications at low temperatures such as a temperature of no greater than about 370° F.